1. Field of the Invention
This invention relates to a solid-state imaging device using a charge-coupled device (CCD), and more particularly to a solid-state imaging device such as a photo-conductive layer overlaid solid-state image sensor.
2. Description of the Related Art
For solid-state imaging devices (e.g., CCD imagers) of double-layer structure where storage diodes, signal reading sections, and signal transferring sections are arranged in a solid-state imaging element chip, which is covered with a photo-conductive film, the opening area of the photosensitive portion through which light comes in can be made wide because of their structure, so that they provide very high sensitivity. Therefore, such solid-state imaging devices are promising as imaging elements for high-sensitivity cameras such as high definition television in the future.
One of the problems of double-layer structure solid-state devices whose photo-conductive films are layered as mentioned above is so-called dark current noise generated in the storage diode portion. The dark current noise (when the image is dark or during a dark time) produces nonuniform output values, which appear as image noise, known as fixed pattern noise (F.P.N.), on the reproduction screen during a dark time.
FIG. 5A is a plan view of a conventional interline transfer (IT) photo-conductive layer overlaid solid-state imaging device, and FIG. 5B is a sectional view of a pixel of the imaging element. The element operates as follows. The light incident on the top surface of the element enters a photo-conductive film 57, where electron and hole pairs are generated holes pairs, thereby converting light into electricity. The photoconductive film 57 of three-layer structure is composed of an intrinsic-type (i.e., i-type) amorphous silicon (a-Si) film on a pixel electrode 56, an i-type amorphous silicon (a-Si) film on the preceding film, and a p-type amorphous silicon carbide (a-SiC) on the immediately preceding film. By an electric field applied to the photo-conductive film by a transparent electrode 58 made of, for example, indium tin oxide (ITO), the signal electrons flow into a pixel electrode, and signal electrons flow via the electrode 61 into an n-type region 53, together with a p-type silicon substrate 51 constituting a storage diode. By turning on a read-out gate 60 also serving as a CCD transfer electrode, the signal electrons accumulated in the storage diode are read via a signal read-out reading portion 55 into an n-type region 54 constituting a buried transfer channel of a vertical charge-coupled device (CCD). An element isolating region 52 of a p.sup.+ -type region is also provided. As shown in FIG. 5A, columns of vertical CCDs are provided in a pixel region 1. The signal electrons read into the vertical CCD are transferred sequentially to a horizontal charge-coupled device 3 provided adjoining the pixel region. The signal electrons transferred to the horizontal CCD are sent by the horizontal CCD to a signal sensing section 4 placed at one end of the horizontal CCD.
In general, dark current flowing into the n-type region 53 forming the storage diode is mainly generated in a depletion layer 59 between the p-type Si (silicon) substrate 51 (or the p-type well region) and the n-type region 53. Especially, in a place where the edge of the n-type impurity diffused region forming the storage diode comes into contact with the p-type Si substrate surface, the depletion layer 59 always exists at the interface of the Si substrate. At the depleted substrate surface, dark current is liable to occur. The value of the dark current tends to vary from pixel to pixel. Because of this, dark current generated in the depletion layer 59 has been a main cause of fixed pattern noise (F.P.N) reproduced images. Fixed Pattern Noise is distinctively found when the image of the subject is dark, namely the amount or the incident light is small. The factors that generate dark current and its effect will be explained in detail.
Conventionally, the potential of the p-type Si substrate 51 and the element isolating region 52 is 0 V, whereas the potential of the n-type region 53 during the accumulation of the signal charge ranges from approximately 10 V to 2 V. The reason why the potential of the n-type region 53 is in the range of approximately 10 V to 2 V is as follows.
Immediately after the signal charge has been read from the n-type region 53 into the vertical CCD channel 54, the potential of the n-type region 53 of the storage diode is determined by the potential applied to the read-out gate 60. Normally, the value is approximately 10 V. After the reading of the signal charge, when the accumulation of the signal is begun, the potential of the n-type region 53 of the storage diode decreases as the amount of signal charges accumulated increases. Finally, it is saturated at the potential applied to the transparent electrode 58. Normally, approximately 2 V is applied to the transparent electrode 58. For this reason, the potential of the n-type region 53 forming the storage diode is set in the range from 10 V to 2 V.
For further detailed explanation, FIG. 6A shows the construction of a conventional element, and FIGS. 6B through 6D show the relationship between the operation of the element and the potential of the n-type region 53 of the storage diode using potential diagrams accompanied by band diagrams. Specifically, FIG. 6B illustrates the potential during the reading of the signal charge into the vertical CCD channel and the state of charge. At this time, the potential of the transparent electrode 58 is approximately 2 V. The potential of the n-type region forming the storage diode is higher than this potential. In this state, the photo-conductive film junction of three-layer structure is in the reverse-biased state.
FIG. 6C depicts the potential diagram during the accumulation of the signal charge and the state of charge, and FIG. 6D depicts the potential diagram at the end of the accumulation of the signal charge, immediately before reading, and the state of charge. Generally, the value of dark current generated in the depletion layer 59 around the n-type region 53 forming the storage diode becomes larger in proportion to the width of the depletion layer 59. The width of the portion in which the depletion layer develops becomes greater as the potential of the n-type region of the storage diode becomes higher. That is, as shown by a graph of actual measurements in FIG. 7B, it is found that, for example, the potential of the n-type region 53 of the storage diode is proportional to the dark current flowing into the n-type region 53.
As described above, the potential of the n-type region 53 forming the storage diode is as high as approximately 10 V when the amount of the incident light is small (i.e., when the amount of the signal charge accumulated is small), so that the width of the depletion layer around the n-type region 53 is also greater. As a result, the value of dark current generated in the vicinity is also greater (cf. value a). On the other hand, when variations in the dark current is less conspicuous or when the amount of the signal charge is great, the potential of the n-type region 53 is small. As a result, the depletion layer around the n-type region 53 forming the storage diode becomes narrower. Consequently, the value of dark current generated in the vicinity becomes smaller (cf. value b). From what has been explained, it can be understood that when variations in the dark current is conspicuous on the reproduced images, the amount of incident light is small (i.e., the amount of the signal charge accumulated in the n-type region 53 is small).
conventionally, however, as seen from the change in the potential of the n-type region 53 constituting the storage diode and the relationship, the dark current generated has had a large value (cf. value a) for a small amount of the signal charge. That is, in a case here the amount of incident light is small so that F.P.N. is conspicuous, more variations in the dark current are liable to occur easily.
As described above, in conventional photoconductive layer overlaid solid-state imaging devices, a lot of dark current generated around the storage diode during the accumulation of the signal charge occurs especially when the amount of incident light is small. Further, during a dark time when the amount of incident light is small, F.P.N on the reproduced image is more conspicuous. Because of the aforesaid multiplier effects, solid-state imaging devices with a conventional construction and operation have chief factors that degrade the quality of the reproduce image.